Abstract: [Objective] This study investigated changes in milk production performance, milk composition traits, and reproductive hormones during estrus in dairy cows. [Method] Thirty-six healthy Holstein cows were selected. Milk yield, body temperature, milk fat, milk protein, lactose, and somatic cell count were recorded during the proestrus, estrus, and metestrus stages. Concurrently, changes in progesterone, estradiol, and prolactin levels were recorded. [Result] During the estrus phase, milk yield decreased, while milk fat and total solids content increased. Progesterone levels decreased, whereas estradiol and prolactin levels increased. [Conclusion] Significant changes in milk production performance and milk composition traits occurred during estrus, particularly the decreased milk yield and increased milk fat and total solids content. These findings indicated significant physiological fluctuations in cows during estrus. Furthermore, the changes in reproductive hormones were closely associated with these alterations in production performance, providing a basis for further optimization of dairy cow management practices.

Key words: estrus, dairy cow, hormone, milk yield